Phytoplankton processes. Part 1: Community structure during the Southern Ocean Iron RElease Experiment (SOIREE)

The first in situ iron-fertilisation in polar Southern Ocean waters, SOIREE (Southern Ocean Iron RElease Experiment) was a mesoscale, 13-day experiment that resulted in a phytoplankton bloom (chlorophyll-a increased six-fold; algal carbon, three-fold). Just prior to the iron enrichment, community chlorophyll-a at the study site was dominated by picoplankton, with an ‘innoculum’ of large diatoms. Iron enrichment resulted in a floristic shift, with increased diatom abundances, and to a lesser extent elevated nanoplankton stocks. Picoplankton stocks initially increased in response to iron supply, but were then grazed down to initial levels by microzooplankton. The chain-forming pennate diatom Fragilariopsis kerguelensis was the dominant bloom species, although <20 μm haptophytes also increased in abundance. Other algal responses to iron enrichment included elevated nanoplankton cell size, increased chain length of Fragilariopsis kerguelensis, and higher cellular chlorophyll-a levels within the pico-, nano- and microplankton. During the SOIREE bloom, chlorophyll-a levels of up to 3.0 mg m−3 were recorded at depth, and averaged 1.8 mg m−3 in the 65-m mixed layer. The floristic shift from a picoeukaryote-dominated community to one dominated by large diatoms was similar to that observed prior to and during algal blooms in the vicinity of open-ocean fronts in the Southern Ocean.